two-year field trial was conducted to evaluate the effect of supplemental irrigation on increasing and stabilizing yield and quality of forage produced by annual ryegrass in southern Portugal. The following four irrigation treatments were tested: rainfed (control), and irrigation up to 25%,50% and 100% of soil water holding capacity. In both years, the importance of irrigation was evident in order to increase and stabilize forage production because even with the lowest amount of water application it was possible to at least double the number of harvests. Also the best response was reached for the 100% treatment. The beneficial effect of increasing water application was more noticeable in the second year. Crude protein and digestible dry matter yields showed similar responses to dry matter yield, except in the first year for crude protein yield, whose values were not significantly different between the three irrigation treatments.